1. Field of the Invention
The present invention relates to a method of boosting chroma for color printing, and, more particularly, to a method of boosting chroma for color printing of an image that includes both flesh-tones and non-flesh-tones.
2. Description of the Related Art
One of the important processes in ink-jet printing is color reproduction. This process includes, among other things, xe2x80x9ctargetxe2x80x9d determination and xe2x80x9cobjectxe2x80x9d generation. The xe2x80x9ctargetxe2x80x9d and xe2x80x9cobjectxe2x80x9d are represented by color profiles in which the color value is specified for each colorant (device value) combination of red, green and blue (RGB) or cyan, magenta, yellow and black (CMYK). The color profile to be reproduced is the target, whereas the color profile on which the color reproduction occurs is the object. For example, the computer monitor profile can be considered the xe2x80x9ctargetxe2x80x9d and the printer profile can be considered the xe2x80x9cobjectxe2x80x9d. With the target and object, it can be determined which printer colorant combination (CMYK) is required for a given monitor colorant combination (RGB) in order to reproduce the target color on the object. Changing the target results in a change in the printer output.
The well-known target of the computer monitor is the computer monitor model. Since the set of all colors that can be produced, known as a xe2x80x9cgamut,xe2x80x9d of this monitor target is much different from the printer gamut, some mapping must be done between the two kinds of gamut. The mapping is typically carried out in an intermediate color space, such as CIELAB, wherein any color value can be expressed in a standard format. In CIELAB color space, luminance (brightness), hue (color) and chroma (saturation) are specified by three-dimensional coordinates (L*, a*, b*). CIE stands for Commission Internationale de l""Eclairage, an international body of color scientists. L* describes relative lightness; a* represents relative redness-greenness, and b* represents relative yellowness-blueness. Luminance is specified by the L* value of the point of interest; hue is specified by the angle that is the arc tan of b*/a*; and chroma is specified by the distance of the point of interest from the L* axis, which can be calculated from the formula [(a*)2+(b*)2]xc2xd.
The mapping can generally give acceptable results for most cases but does not always yield desired outputs. For example, the surround of a picture of a human in a photo image reproduced on an ink jet printer may not be vivid enough. However, if the chroma is boosted to make the surround more colorful, the flesh-tones, or skin-tones, of the human picture become unrealistically vivid.
What is needed in the art is a method of smoothly transforming two color domains, such as flesh-tones and non-flesh-tones, in different ways so as to make the surround in a human picture more colorful without making the skin-tones of the human picture unrealistically vivid.
The present invention provides a method of making the non-flesh-tone domain of photo images more vivid while changing the flesh-tone domain to the desired colors.
The invention comprises, in one form thereof, a method of transforming two color domains of an original target color profile in a universal color space. Information characterizing the original target color profile is received. A non-flesh-tone domain chroma boost curve is constructed. The non-flesh-tone domain chroma boost curve is applied to the original target color profile to generate an intermediate target color profile. A flesh-tone domain boundary in the intermediate target color profile is determined. A flesh-tone domain chroma boost curve is constructed. A new target color profile is established by applying the flesh-tone domain chroma boost curve to the intermediate target color profile.
An advantage of the present invention is that the chroma surrounding human skin tones in an image can be boosted without boosting the skin tones to a degree that the skin appears unrealistically vivid.
Another advantage is that a transition between the two domains is smooth and produces no artifacts.